(1) Field of the Invention
The present invention lies in the technical field of power plants for rotary wing aircraft. The invention relates to a method of managing an engine failure on a rotary wing aircraft having a hybrid power plant with at least two fuel-burning engines. The invention also relates to a rotary wing aircraft having such a hybrid power plant and including a device for managing an engine failure.
(2) Description of Related Art
A rotary wing aircraft is conventionally provided with at least one main rotor for providing it with lift and possibly also propulsion, and generally with a tail rotor in particular for opposing the yaw torque exerted by the main rotor on the fuselage of the aircraft and also to enable the yaw movements of the aircraft to be controlled.
In order to drive the main rotor and the tail rotor in rotation, the aircraft has a power plant that may include one or more engines.
A distinction is drawn between aircraft of the “single-engined” type, in which the power plant has only one engine for driving the main rotor and the tail rotor, and aircraft of the “multi-engined” type, in which the power plant has at least two engines for this purpose, with this type including in particular aircraft of the “twin-engined” type, where the power plant has two engines.
It should be observed that throughout the present specification, the term “engine” is used to designate a fuel-burning engine such as a turboshaft engine or a piston engine suitable for use in such a power plant. The term “engine” should be contrasted with the term “electric motor” designating a motor driven by electrical power.
Single-engined aircraft present advantages over multi-engined aircraft, such as reasonable cost, reduced maintenance operations, and relatively low fuel consumption. Nevertheless, such single-engined aircraft also present drawbacks.
In the event of damage to the single engine, the power plant and consequently the single-engined aircraft, presents performance that is degraded, possibly to such an extent as to be incapable of driving the main rotor and the tail rotor. Under such circumstances, the pilot of the aircraft must enter into an autorotation stage of flight, and must then perform an emergency landing, with the main rotor being in autorotation. This constitutes a difficult operation in the piloting of aircraft and in particular of single-engined aircraft, and is one of the main reasons for aircraft of this type having a reduced flight envelope and restricted use.
The flight envelope and the missions that can be authorized for single-engined aircraft are restricted by the certification authorities that issue flight authorizations. For example, in Europe, a single-engined aircraft is not permitted to overfly a large city.
In contrast, aircraft having at least two engines, such as twin-engined aircraft, make it possible firstly, with both engines operating simultaneously, to achieve improved performance, in particular in terms of transportable payloads and maximum range, and secondly, in the event of one of the engines failing, to make the flight capacity of such aircraft safer since each of the engines can take over from the other. As a result, flight restrictions such as overflying a large city, do not apply for this type of aircraft.
Nevertheless, the flight capacities authorized for such aircraft can still be limited compared with their maximum capacities in order to ensure some minimum level of safety in flight. In the event of an engine failing, it is sometimes necessary for a twin-engined aircraft to be able to continue flying on one engine, and thus with reduced performance. Certification authorities take account of such reduced performance while using a single engine when defining the capacities that are authorized for such aircraft, such as maximum weights on takeoff and landing.
In order to avoid pointlessly limiting the flight envelope of such aircraft, it is appropriate to have a level of power that is available with only one engine that is sufficiently high to be able to accommodate the risk of a failure. Certain twin-engined aircraft thus have their engines dimensioned so as to enable the aircraft to be used with only one of its engines, and they are thus over-engined in normal flight, i.e. when both engines are operating together. The main consequences of this are an increase in the fuel consumption of the aircraft, which can be made worse by the increase in its empty weight.
A multi-engined aircraft, such as a twin-engined aircraft having two turboshaft engines, for example, has various operating ratings. Firstly, when both engines are operating, the aircraft operates under all engines operative (AEO) ratings. Each engine then has a maximum continuous power (MCP) rating that can be used with no limit on time and a maximum takeoff power (max TOP) rating in which it delivers power greater than the MCP, but which is generally limited to being used for 5 minutes on civil aircraft, this rating being intended specifically for stages of takeoff and landing, and also for short duration hovering flight.
In the event of an engine failure on a multi-engined aircraft, a failure of at least one engine leads to power from that engine being lost in part or in full. In such an aircraft, the probability of having a failure simultaneously on a plurality of engines is relatively low, in compliance with the regulations that are in force. Generally, in the event of an engine failure, at least one other engine remains fully operational and the engine(s) that remain operational is/are capable of providing the aircraft with drive.
For a twin-engined aircraft having two turboshaft engines, the aircraft then operates in a degraded mode with a single engine that remains operational and that presents various contingency overpower ratings labeled with the acronym OEI for “one engine inoperative”. For the turboshaft engine that remains operational, these ratings are conventionally:
a first contingency rating referred to as OEI 30″, associating a supercontingency power level with a usable duration of about thirty consecutive seconds;
a second contingency rating referred to as OEI 2′, associating a maximum contingency power level with a usable duration of two minutes; and
a third contingency rating, referred to as OEI Cont, associating an intermediate contingency power level with a usable duration that is unlimited, e.g. until the end of the flight.
It should be observed that some engines propose a single contingency rating, referred to as OEI 2′30″, that replaces the contingency ratings OEI 30″ and OEI 2′, with this single rating being usable for a duration of two minutes and thirty seconds.
The intermediate contingency rating OEI Cont is used in particular for providing cruising flight, whereas the OEI 30″ power rating and the OEI 2′ power rating or the single OEI 2′30″ power rating are for use in performing particular maneuvers such as avoiding an obstacle, hovering, or landing.
Furthermore, it is possible to use the OEI 30″ power rating only two or three times, depending on the aircraft and until it lands, whereas the OEI 2′ power rating and the OEI 2′30″ power rating can be used several times over for a total accumulated time not exceeding a length of time as predetermined by the manufacturer of the aircraft and as specified in maintenance manuals, e.g. ten minutes.
The extra power delivered by the single engine that is available after a failure is obtained by stressing the engine beyond its normal operation and its nominal limits. As a result, such operation needs to be followed by special maintenance with associated costs that can be significant.
Furthermore, the operating durations for the OEI 30″ power rating, for the OEI 2′ power rating, and for the OEI 2′30″ power rating are limited to avoid leading to major, and possibly also immediate, damage to the turboshaft engine or to the power transmission means, such as the main gearbox or “MGB”.
Other solutions have been investigated for delivering such extra power, in particular in order to be able to avoid overdimensioning the engines, but none have yet been applied industrially. For example, the extra power may be obtained by injecting water or a water/alcohol mixture into the air inlet of the engine or by cooling hot parts of the engine by circulating air or water. It is also possible to inject a self-igniting fuel downstream from the combustion chamber of the engine or indeed to use an auxiliary power unit including a fuel-burning engine, e.g. running on kerosene or hydrazine.
The OEI 30″, OEI 2′, OEI 2′30″, and OEI Cont power ratings are controlled by an electronic control unit of the engine. Each engine is connected to such a control unit that is commonly referred to as an electronic engine control unit or “EECU”.
This type of EECU is present on most aircraft for controlling the operation of the engine(s). In certain aircraft, the EECU is replaced by an engine computer, commonly referred to by the acronym FADEC for “full authority digital engine control”. Such a FADEC engine computer has greater authority than an EECU and thus limits pilot intervention in controlling the engine(s).
One solution envisaged for improving the performance of aircraft is to use a “hybrid” power plant.
As in the automobile field, a “hybrid” power plant has at least one fuel-burning engine and at least one electric motor, the driving power from the hybrid power plant being delivered either by the engine on its own, or by the electric motor on its own, or indeed by both together. For the particular circumstance of twin-engined aircraft, a hybrid power plant has two engines that operate simultaneously together with at least one electric motor.
For example, document FR 2 952 907 describes a hybrid power plant used on a single-engined aircraft having a single engine together with a first electric motor that is mechanically connected to the main rotor of the aircraft, and a second electric motor that is mechanically connected to the tail rotor. That hybrid power plant also has a set of batteries for storing the electrical energy needed for electrically powering the two electric motors.
Those electric motors can act in addition to or as a replacement for the engine in order to drive the main and tail rotors. Furthermore, those electric motors may operate in generator mode for transforming mechanical power into electrical power, and also for slowing down the rotors or indeed the engine.
Document FR 2 962 404 describes the electrical architecture of a hybrid power plant for a rotary wing aircraft. That power plant has at least one engine and at least one electric motor together with a main electricity network and an auxiliary electricity network. The main electricity network is for providing the general electricity power supply of the aircraft, while the auxiliary electricity network is dedicated to the system for hybridizing the hybrid power plant.
Document DE 10 2007 017332 describes an aircraft having a propulsion unit constituted by a propeller, an engine driving the propulsion unit, and an electric machine. The electric machine co-operates with the engine, operating either as an electric motor or as an electricity generator. The electric motor thus delivers additional power to the engine, lying in the range 15% to 35% of the power of the engine.
Furthermore, document FR 2 914 697 relates to a turboshaft engine, in particular for a helicopter, having a compressor and a free turbine, together with an electric motor connected to the compressor. The electric motor enables the turboshaft engine to have better acceleration capacity while maintaining the same surge margin by providing an additional quantity of rotary kinetic energy to the compressor during a stage of accelerating the turboshaft engine.
Finally, document FR 2 961 767 describes a method of managing an electric circuit of a vehicle that has an engine for driving a propulsion member and a reversible electric machine connected to at least one battery. That method enables the electric machine to operate in generator mode in order to recharge the battery, or else in motor mode in order to drive the propulsion member.
In addition, the technological background includes in particular the documents FR 2 947 006, DE 10 2010 021025, and FR 2 735 239.
However, one of the major drawbacks of using electric motors is storing the electrical energy needed to operate them. Several solutions exist for storing this electrical energy, such as batteries, thermal batteries, or supercapacitors, but each of them has its own constraints.
For example, batteries are heavy or indeed very heavy if a large quantity of electrical energy is to be stored, whereas supercapacitors are capable of delivering a high level of electrical power, but only during a very limited length of time. Furthermore, thermal batteries are for single use only and the length of time they operate after being activated is limited.
Whatever the means used for storing electrical energy, the quantity of electrical energy that is available remains limited, while the weight of the electrical energy storage means can be considerable.
Thus, any improvement in performance that it might be possible to obtain by using one or more electric motors within the power plant of an aircraft suffers from various limitations associated with storing electrical energy. For example, it is necessary to find a balance between the improvement in performance of the hybrid power plant and the increase in weight caused by using such electrical energy storage means that are needed to operate the electric motor(s).